Fragmentation of heme and hemin+ with sequential loss of carboxymethyl groups: A DFT and mass-spectrometry study

Density functional B3LYP calculations with the 6-31G* and mixed 6-31G* (Fe) + 6-31G (C, H, N, O) basis sets have been performed to study the electronic and geometric structure, thermodynamic stability, and vibrational frequencies of the heme molecule FeC34H32N4O4, its ion FeC 34 H 32 N 4 O 4 + , and products of their fragmentation with sequential elimination of two carboxymethyl groups in the electronic states with different spin multiplicities. The fragmentation energies were refined with the larger 6-311++G** basis set. Results of the calculations are compared with available experimental data. The trends in the aforementioned properties with a change of the multiplicity, molecular charge, and the number of external carboxymethyl ligands present have been analyzed.